High frequency characteristics in low temperature pseudo-heterojunction bipolar transistors

A pseudo-HBT (heterojunction bipolar transistor) having a cutoff frequency (f_T) twice as high as that of a conventional bipolar transistor at 87 K is demonstrated. This is the first bipolar transistor that has a higher f_T below 100 K than at room temperature. The low-impurity-concentration graft base reduces the electron injection into the graft base, resulting in a high cutoff frequency. The impurity concentration of the graft base (extrinsic base) must be kept lower than that of the intrinsic base. In addition. the base profile must be kept as abrupt as possible to prevent the reduction of effective bandgap narrowing in the base